This is when a L to R shunt reverses to become R to L because of compensatory pulmonary vascular hypertrophy.

Initial L to R shunt increases blood flow to pulmonary circulation leads to pulmonary HTN forcing the pulmonary vasculature to hypertrophy. This increases pulonary resistance leading to a reversal in shunt. Later R to L shunt causes the "late cyanosis" in L to R shunt defects.